1. Field
The present general inventive concept relates to an electrophotographic toner, and more particularly, to a toner to develop an electrostatic latent image.
2. Description of the Related Art
In general, electrophotographic imaging involves the following processes: uniformly charging a surface of an electrostatic latent image carrier, exposing the surface of the electrostatic latent image carrier to form an electrostatic latent image thereon, adhering toner to the electrostatic latent image to visualize the electrostatic latent image, transferring a resulting toner image onto a recording medium such as paper, cleaning the electrostatic latent image carrier to remove the toner remaining thereon, erasing charges from a surface of a photoreceptor to lower electrical characteristics, and fusing the toner image onto a recording medium by heat or pressure.
To obtain electrophotographic toner with appropriate characteristics, technologies of controlling the shape and surface of toner particles have become more important. The faster the printing speed of a printer, the more frequently the toner is subjected to a shearing force. Thus, stronger durability is required for toner. To realize a compact, environmentally-friendly printer, the amount of untransferred toner may be reduced. To this end, improvements in charge uniformity and transfer efficiency (transferability) of toner are advantageous. Improving charge stability, transfer efficiency, and cleaning ability of toner are effective in obtaining high-quality printed images.
To provide toner particles with high charge uniformity, high charge stability, high transfer efficiency, and high cleaning ability, surface characteristics of the toner need to be improved. One of the important factors affecting the surface characteristics of toner is an external additive adhered to surfaces of the toner particles. A primary function of the external additive is preventing adhesion of the toner particles to each other to maintain flowability of the toner particles. The external additive may also affect charge uniformity, charge stability, transfer efficiency and cleaning ability of the toner. For example, silica powder or titanium oxide powder is generally used as an external additive.
Conventional external additives are known to be ineffective in terms of charge uniformity. For example, fumed silica particles, as a most widely used external additive, have strong negative polarity. For this reason, excess charge-up may frequently occur with the use of toner that includes fumed silica as an external additive.
To prevent excessive frictional charging resulting from an excess charge-up due to the use of fumed silica, using titanium oxide particles as an external additive has been suggested. However, titanium oxide has a low electric resistance and an effective charge exchangeability, and may easily produce a reverse- or weak-charging toner. Accordingly, using titanium oxide as an external additive may lower charge uniformity of the toner.
Silica particles may be porous and have hydrophilic surfaces. When a toner including highly porous, highly hydrophilic silica particles as an external additive is used in a high-temperature, high-humidity environment, the toner may not be smoothly charged due to the absorption of excess water that serves as an electric conductor. On the other hand, a toner including silica particles as an external additive tends to be excessively charged in a low-temperature, low-humidity environment, and thus may have ineffective charge stability due to environmental condition changes. Consequently, ineffective toner concentration reproducibility and background staining in a high-temperature, high-humidity environment, or electrostatic staining of an image at low temperature and low humidity may result.
To address a reduction in environmental charge stability caused by moisture, silica particles or titanium oxide particles, each treated with a surface treating agent such as hydrophobic silicone oil or hydrophobic silica coupling agent may be used as an external additive. However using external additive particles surface-treated with such a surface treating agent may enhance cohesiveness of toner particles, and may instead sharply reduce flowability of the toner particles.
In preparing fumed silica particles, the silica particles are frequently apt to form agglomerates, which may lower dispersibility of the fumed silica particles. Using such an external additive with an inherently poor dispersibility may also lower flowability, anti-caking ability, fusability, and cleaning properties of the resulting toner.
To prevent the aggregation of the fumed silica, sol-gel silica may be used. Sol-gel silica powder refers to silica powder prepared using a sol-gel method. For example, sol-gel silica powder may be prepared by hydrolysis and condensation of alkoxy silane in an organic solvent in the presence of water, and removing the solvent from a silica sol suspension resulting from the condensation. Sol-gel silica powder prepared by a sol-gel method may consist of spherical silica particles having a uniform particle size. Conventional sol-gel silica particles have almost perfect spherical shapes. Using silica particles having a sphericity near to 1 as an external additive may deteriorate cleaning properties of the toner that includes the external additive.
Recently, use of small-diameter toner has been steeply increased to provide high image quality. However, using such inorganic particles in preparing small-diameter toner particles does not ensure sufficiently good performance. The smaller the toner diameter, the more ineffective the flowability of the toner particles may become, and the greater the number of inorganic particles may be required as an external additive. The external additive is exposed to friction against a supply roller and a blade or due to stirring within a developing unit during electrophotography. Stress exerted on the toner particle during this process may cause the external additive to be separated from the toner surfaces or to be buried in the toner surfaces. As a result, the toner may have ineffective flowability, may be unable to be smoothly supplied in an electrophotographic imaging system, and may have increased adhesion to a developing roller, resulting in sharp reductions in development characteristics and durability.
The smaller the toner particles become, the higher the charge quantity may become, and the higher the adhesion of the toner particles to a developing roller may become. Consequently, development characteristics of the toner may be deteriorated. The adhesion of the toner particles to a photoreceptor may also be increased, leading to deterioration in transfer characteristics of the toner. The higher the charge quantity becomes, the more the toner may be apt to cause a charge-up in a low-temperature, low-humidity environment. To prevent a charge-up and to improve development characteristics and transfer characteristics of a toner, there is a need to reduce the amount of charges of the small-diameter toner.